AbstractRadical cyclopolymerization ofN‐propyldimethacrylamide (PDMA) was investigated. Kinetic orders of polymerization rate with respect to the monomer and the initiator were 1,0 and 0,5, respectively, at a monomer concentration greater than 1,6 mol/l. Absolute values of rate constants of propagation and termination reactions were determined by the rotating sector method in bulk and in 50% propionitrile solution at 30°C. Poly(PDMA) consists of 5‐membered cyclic repeating units almost exclusively. The rate constants for addition of poly(PDMA) radical to vinyl monomers and those for addition of polymer radicals to PDMA were also evaluated. One the basis of the rate constants obtained, the rate determining step of PDMA propagation was deduced to be the intermolecular addition of the growing radical to PDMA monomer. The growing radical of PDMA, formed by intramolecular addition to the carbon‐carbon double bond of the same molecule, was found to be as reactive as polymer radicals from reasonance stabilized monomers. The 5‐membered ring formation was highly favored by the conformation of PDMA in which coplanarity of the newly formed bond and the semi‐occupied orbital of the new radical center is readily achieved. Relatively low reactivity of poly(PDMA) radical was accounted for by delocalization of the unpaired electron on the coplanar three carbon atoms. Radical addition of a 2‐tertiary thiol to PDMA solely yielded the substituted succinimide. The 5‐membered ring formation in PDMA polymerization and in the addition reaction was ascribable to the favorable confor